The invention relates to a device for supporting a trailer of a semitrailer truck comprising a vertical spindle drive for adjusting the height of the support device, a horizontal, particularly a crank-handle-driven input shaft, and an angle drive for redirecting the torque of the input shaft to the spindle drive. The angle drive comprises at least two gears that are spaced at a distance from one another for gear positions of different gear ratios, which can be optionally engaged with a ring gear of the input shaft by horizontally displacing the input shaft.
The support devices usually have several gears with different gear ratios, which are optionally selected by shifting. These are typically two-speed gears in which one gear, the so-called low gear, is used for lifting loads and the second gear, the so-called overdrive gear, for quick telescoping of the lifting jack without load or with a small load.
Such a support device is described in German Laid-Open Publication DE 19 836 635 A1, which must be considered definitive. The prior art support device, in addition to the two gear positions, also has a neutral position. In this position, torque transmission between input shaft and output shaft is interrupted.
Consequently, neither driving nor braking torques can be transmitted. Thus, if the driven shaft is stationary the drive shaft can rotate, and vice versa. In a support device, the neutral position is used to make it easier to find the gear positions by rotating the crank. However, if the support device is left in neutral position during travel, i.e. with the inner legs cranked up, the absence of a braking torque has a negative effect. Due to vibrations occurring during travel, the inner legs can descend and touch the ground. This can cause serious damage and even result in the loss of parts on the road. Although the manufacturers point out that the device must be put in gear during travel, these instructions are often not observed in daily practice.
U.S. Pat. No. 5,238,266 discloses a support device with a gear without a neutral position. This ensures that the device is always in gear and the gears thus prevent an interior leg from descending. A substantial drawback of this support device is that during shifting between gears there is a position in which both gears are engaged simultaneously. This not only affects the ease of shifting but may also cause damage to the components that are engaged.
The object of the invention is to provide a support device whose gears are equipped with a neutral position with its advantages. Any unintended descending of an inner leg of the support device is to be prevented.
According to the invention, this object is attained by a support device in which the ring gear of the input shaft has two longitudinal sections of different diameters but with uniform tooth pitch throughout. In other words, one section has a normal diameter with full tooth formation, the axial length of which is less than the distance between two gears, to define a neutral position for the input shaft, and an extension section with radially reduced tooth length, which when the input shaft is in neutral position protrudes into a gear. This reduced length of the teeth of the extension section is dimensioned in such a way that in neutral position when the gear is stationary the input shaft is rotatable, but when the input shaft is stationary the gear is prevented from rotating because the teeth with reduced length engage.
This design makes it possible to shift between the two gear positions through a neutral position. This prevents the crank drive, which is displaceable in axial direction, from simultaneously engaging with its gear in both gear positions.
In neutral position, the gear with reduced tooth length in the extension section rotates past the stationary external gear teeth of the drive shaft for the low gear position when the crank drive rotates.
During travel, however, the inner leg may move downwardly inside the exterior space due to its own weight as a result of shocks and vibrations. This is prevented according to the invention by blocking the external gear teeth of the drive shaft for the low gear, which rotate as the inner leg moves downwardly, through the reduced tooth length of the extension sections.
In a preferred embodiment, the angle drive has exactly two gears that are spaced at a distance from one another for gear positions of different gear ratios. The gear with the smaller diameter forms an overdrive position together with a gear driven by the crank drive, while the non-displaceable gear with the larger diameter can be brought into direct engagement with the gear of the crank drive by axially displacing the crank drive.